1. Field of the Invention
The present invention relates to a rotation speed search apparatus for an induction motor and its method, and more particularly, to a rotation speed search apparatus for an induction motor and its method that are capable of optimally increasing an output voltage of an inverter to reach a target voltage in any circumstances by increasing the voltage of the inverter by using an actual current flowing at a motor.
2. Description of the Background Art
In general, when an induction motor is driven through an inverter, it is a frequent occurrence that the inverter needs to be started while the induction motor is being driven (for instance, in case of an instantaneous power failure, re-starting after broken down or in case that the induction motor is connected to the same load in parallel). In this case, in order to drive the induction motor through the inverter without causing any excessiveness, a voltage and a frequency should be suitably inputted from the inverter to the motor in the ratio of V (rated voltage)/F (rated frequency) corresponding to a rotation speed of the induction motor.
An apparatus and method for searching a rotation speed of an induction motor by controlling an output voltage and an output frequency of the inverter for a normal operation of the inverter will now be described with reference to FIG. 1.
FIG. 1 is a block diagram of a rotation speed search apparatus for an induction motor in accordance with a conventional art.
First, in case that a motor of an induction motor is normally accelerated, a frequency generator 11 receives a target frequency according to user's input. The frequency generator 11 then generates the target frequency as a frequency having a ramp waveform during pre-set accelerating time and outputs it.
The output frequency outputted from the frequency generator 11 is converted into a frequency suitable for generating a PWM signal in an output frequency converter 13 and inputted to a PWM (Pulse Width Modulation) generator 15.
The output frequency outputted from the frequency generator 11 is inputted to a voltage generator 12 and then converted into a voltage having a frequency suitable for generating a PWM signal in an output voltage converter 14 and inputted to the PWM generator 15. The voltage inputted to the output voltage converter 14 has a value of input frequency * (V/F ratio).
Thereafter, in order to start the induction motor, the PWM generator 15 receives the output voltage and the output frequency to generate a PWM signal and outputs the PWM signal to the motor to start it.
When the motor is normally started by using the above-described method, the V/F ratio is constantly maintained.
As afore-mentioned, the rotation speed search method signifies searching a rotation speed of the motor and starting the motor in a state that the speed of the motor is not ‘0’ due to an abnormal situation such as an instantaneous power failure.
The motor speed search method in an abnormal situation will now be described in detail.
To begin with, if a motor speed is to be searched in an abnormal situation, a target frequency is inputted directly to the PWM generator 15 like ‘b’ without passing through the frequency generator 11. In other words, if a user's command (that is, a target frequency) is 60 Hz, an output frequency becomes 60 Hz. At this time, assuming that 220V, suitable to 60 Hz, is applied to the motor (that is, only for a 220V class motor), that is operating at around 60 Hz, there is no problem. But if the rotation speed of the motor is slow due to a power failure or the like and 220V is directly applied thereto, an excessive current flows from the inverter to the motor, which can damage the motor.
Therefore, in order to solve the problem, in the conventional art, upon the occurrence of the problematic situation, even if the output frequency is 60 Hz, it is controlled such that the output voltage is gradually increased from ‘0’ so that 220V can be applied to the motor after a certain acceleration time.
FIGS. 2A to 2C are graphs showing an operation interval for searching a rotation speed of the induction motor.
As shown, an operation interval for searching a rotation speed of the induction motor is divided into three cases: first case where power failure occurs during operation of the motor and then power is recovered (FIG. 2A); second case where utility power is OFF and the motor is operated again (FIG. 2B), and third case where the motor being rotated is promoted to a target rotation speed (FIG. 2C).
In this manner, the induction motor speed searching is performed by controlling the voltage and the frequency, outputted from the inverter, so that the inverter can be normally operated when a rotation speed of the motor is unknown.
FIGS. 3A and 3B are graphs showing a method for simply controlling an output voltage of the inverter in order to searching a rotation speed of the induction motor in accordance with the conventional art.
As shown, the output frequency of the inverter is maintained to have a certain target frequency over time (FIG. 3A) while the output voltage of the inverter is increased to have a certain tilt over certain acceleration time, so as to reach a target voltage (FIG. 3B).
On the assumption that an output voltage of the inverter is ‘n’, it can be expressed that the output voltage (n)=an output voltage prior to certain time (n−1)+dv (dv=target voltage/acceleration time). The output voltage is increased with a certain tilt from voltage ‘0’ to the target voltage during certain acceleration time.
However, the conventional method in which the output frequency of the inverter is fixed to be constant and a speed of the induction motor is searched and controlled by using only the output voltage has the following problems.
First, in case of a load with a strong inertia, if acceleration time is short, there occurs an overload trip, and with the big load, if acceleration time is lengthened, the speed of the motor drops to ‘0’. Therefore, a user should directly set the acceleration time of the induction motor.
In addition, in case of a big load, the output frequency of the inverter applied to the motor becomes the target frequency and the slip of the motor nears ‘0’ to generate a little torque, so that the induction motor is not started properly.
Moreover, if the target frequency is lower than a current operation speed of the induction motor, the induction motor is in a regenerative region. Then, the induction motor won't be operated with a simple voltage control method of the inverter.